1. Field of the Invention
This invention relates to pulsed radar systems, and more specifically to a radar system and method of phase noise compensation capable of detecting micro movement.
2. Description of the Related Art
Doppler radar is used to measure both the range to target and the “doppler” movement of the target. Pulsed radar system emit pulses and A/D sample the return pulses to extract range and movement of the target. The pulses may be single-frequency pulses or more recently stepped frequency pulses. Typically, the pulses are in the RF band, approximately 4-100 nsec. Experimental “impulse” systems transmit very short pulses, less than 1 nsec to achieve instantaneous high range resolution. In each of these cases, the bandwidth of the A/D converter is quite high, 100 Mhz range for pulsed RF and 1 Ghz for impulse systems. Such high bandwidth A/D converters typically have a spur free dynamic range of no better than 60 dB and 30 dB, respectively.
Pulsed radar systems use an oscillator to generate the signals to form the transmission pulses. The frequency of the oscillator drifts over time. As a result, there is a phase shift between the frequency used to generate a transmission pulse and the frequency of the receiver that processes the return pulse solely due to this drift. This phase shift manifests itself as “phase noise”. The level of phase noise is greatest near DC and increases as the range to target increases; the oscillator has a longer time to shift and thus will shift to a greater degree.
Pulsed radar systems do not compensate for phase noise other than to stabilize the oscillator to the degree possible. The reasons for this are threefold. First, the phase noise is typically overshadowed by the system noise associated with the A/D converter and receiver. Reducing the phase noise would have negligible effect on the overall noise level of the receiver. Second, until recently most radar applications were directed to targets at long stand-off ranges having a large radar cross-section and large Doppler frequency. For example, airplanes, missiles, and fast moving vehicles would produce a large return at a large Doppler shift from DC. These types of signatures typically lie well above the phase noise and above the system noise and can be detected using well known processing techniques. Finally, there is no known technique for effectively compensating for phase noise.
More recently efforts have been made to apply pulsed radar to urban environments or an urban battlefield. In these environments the stand-off range is much shorter, typically 100 m to 1 km, and the target signatures are much weaker. Instead of fast moving aircraft or missiles the targets are humans or slow moving vehicles, which present a much smaller radar cross-section and a much smaller Doppler shift (where phase noise is its greatest). Such attempts have been unsuccessful because the target signatures associated with “micro movement” are buried in the overall system noise and even the phase noise. The phase noise component can be alleviated somewhat at very short stand-off ranges. This may be adequate in non-hostile environments but is not acceptable under battlefield conditions.
There is a demonstrated and ongoing need for a pulsed radar system that reduces both A/D noise and phase noise sufficient to accurately detect micro movement in an urban environment or battlefield.